The invention relates to a solution masterbatch of rubber and fillers for use in compounding rubbers.
When producing elastomeric compositions for use in rubber articles, such as tires, power belts, and the like, it is desirable that these elastomeric compositions are easily processable during compounding and have a high molecular weight with a controlled molecular weight distribution, glass transition temperature (Tg) and vinyl content. It is also desirable that reinforcing fillers, such as silica and/or carbon black, be well dispersed throughout the rubber in order to improve various physical properties, such as the compound Mooney viscosity, modulus, tangent delta (tan δ), and the like. Rubber articles, especially tires, produced from vulcanized elastomers exhibiting these improved properties will have reduced hysteresis, improved rolling resistance, snow and ice traction, wet traction, improved wear and improved fuel economy for vehicles equipped with such tires.
The compounding of rubber by conventional means in mixers, such as the Banbury mixer, requires a number of process stages in which raw rubber material in the form of bales undergoes intimate mixing with reinforcing fillers, oils, plasticizers, vulcanization auxiliaries, and other common ingredients used in rubber compounding. Between the process stages, the mixtures are generally cooled on a batch-off system and then may be put into intermediate storage, prior to curing and tire building.
Such conventional rubber compounding processes are time- and energy-consuming, especially when silica is used as a reinforcing filler. In particular, silica particles contain polar silanol groups on the surface and tend to self-associate and reagglomerate extensively after compounding, leading to poor silica dispersion and a high compound viscosity. The strong silica filler network results in a rigid uncured compound that is difficult to process in extrusion and forming operations. Therefore, silica is typically chemically reacted with an organosilane silica coupling agent during the compounding process, requiring more time and energy for processing than compounds employing carbon black as a filler.
To address these problems, efforts have been made to predisperse fillers in dried masterbatches made from elastomer dispersions and aqueous dispersions of silica, prior to their use in compounding processes. Although masterbatches providing a mixture of organic solution polymerized elastomers and carbon black have been successful because of the hydrophobic nature of both the elastomer and the carbon black, the use of silica as a filler in a masterbatch has proven to be problematic because silica particles have hydrophilic surface characteristics, resulting in a two-phase system in which the silica is in an aqueous solution and the elastomer is in an organic solvent. Therefore, the use of silica in masterbatches has, for the most part, been limited to (a) mixing the silica with emulsion polymerized elastomers in aqueous solution, (b) hydrophobation of the silica prior to mixing with an elastomer solution-polymerized in an organic solvent, and/or (c) compatibilizing the elastomer for mixing with the silica in an aqueous phase. None of these processes for preparing silica-filled masterbatches has been satisfactory, in that a process such as (a) does not allow for the use of solution polymerized elastomers in organic solvents, and processes such as (b) and (c) have required time-consuming and expensive chemical reactions to compatibilize the reactants.
Therefore, there is still a need to provide solution masterbatches containing natural rubbers or solution polymerized elastomers in organic solvents, and silica and other fillers, that are not complicated or expensive to prepare and that have excellent filler dispersion, to produce rubber compounds having low hysteresis properties and improved processability.